1. The Field of the Invention
The present invention relates to a technique for a pressure reducing valve assembled in a common rail type fuel injection system, in particular relates to the technique to accurately reduce a pressure of fuel accumulated in the common rail using the pressure reducing valve.
2. Description of the Prior Art
As fuel injection systems for internal combustion engines, a common rail type fuel injection system has been known. In the common rail type fuel injection system, high pressure fuel is accumulated in an accumulation device connected to injection pipes each being directed to an individual injector on respective ends assigned to respective engine cylinders. Such a fuel injection apparatus for a diesel engine is generally referred to as a “common rail” connecting between a high pressure pump pipe and injection pipes. Injectors are triggered individually by an engine control system. In such an accumulator fuel injection system for the internal combustion engines or a common rail fuel injection system for the diesel engines, high pressure fuel is accumulated in the fuel accumulating device (i.e., the common rail). One of the characteristic features of these systems is that the pressure of fuel accumulated in the fuel accumulating device is extremely high, which for example is about 150 Mpa or more in some diesel engines.
A typical fuel injection system for internal combustion engine comprises a fuel accumulating device (a common rail for a diesel engine), which is connected to an injection block that includes injection pipes and a plurality of injectors. The fuel accumulating device is further connected to a high pressure pump for pumping high pressure fuel through a high pressure pump pipe. The high pressure pump is a high pressure fuel supplier to the fuel accumulating device after fuel is drawn from a fuel tank and is highly compressed. The fuel accumulating device is provided in the interior thereof with an accumulation chamber for accumulating high pressure fuel. Injectors are mounted on respective cylinders of the engine for injecting fuel into the respective cylinders. Therefore, fuel is normally transported from the high pressure pump to each of the injectors in the respective cylinders of the engine. The high pressure pipe are connected to the respective cylinders via the high pressure pump pipe, the accumulation chamber of the fuel accumulating device, and the respective injection pipes. The injectors are connected at a downstream end of the plurality of injection pipes branching out from the fuel accumulating device. Each injector includes essentially a fuel nozzle or atomizer and a solenoid valve. The solenoid valve is energized by an electronic control unit (ECU) via an engine drive unit (EDU). The ECU is configured to activate the injection of fuel into each cylinder of the internal combustion engine in response to signals not only from an accelerator pedal driven by a vehicle driver but also from sensors mounted on the vehicle. The sensors monitor engine conditions. The engine conditions include such members as crank speed, cam phase, air temperature, coolant water temperature, boost pressure, and air mass. When the solenoid valve is de-energized, the injection stops. Fuel leaked from the injectors is returned to a fuel tank via a relief pipe.
In addition to the above mentioned typical fuel injection system for internal combustion engines, in particular, for diesel engines, there is a further known a common rail type fuel injection system in which a pressure reducing valve is provided with a fuel accumulating device which discharges the high pressure fuel stored in the fuel accumulating device for reducing the internal pressure of the common rail. Such a system having the fuel accumulating device with a pressure reducing valve is disclosed, for example, in Japanese Unexamined Patent Publication 2005-139928, corresponding to U.S. Pat. No. 6,966,300 to Fukuda.
In the common rail type fuel injection system according to Fukuda, a pressure reducing valve and a pressure sensing means for sensing a fuel pressure in the fuel accumulating device are included in addition to the typical devices of the above mentioned typical common rail type fuel injection systems. Both of the pressure reducing valve and the pressure sensing means are generally installed in the fuel accumulating device, but alternatively, only the pressure reducing valve can be installed in the fuel accumulating device, and the pressure sensing means can be externally connected to the fuel accumulating device. The pressure reducing valve adjusts a degree of opening of a drain passage, through which the fuel accumulated in the fuel accumulating device is drained. A maximum draining rate of the pressure reducing valve for draining the fuel accumulated in the fuel accumulating device is greater than a maximum feed rate of fuel. The fuel is fed from the high pressure pump to the fuel accumulating device via the high pressure pump pipe. The degree of opening is controlled by the electronic control unit (ECU) based on results of continuously measured fuel pressure in the fuel accumulating device by the pressure sensing means so that the fuel pressure in the accumulating device reaches to and agrees with a predetermined target value of the fuel pressure.
Thus, if a target value of fuel pressure in the fuel accumulating device is set near the upper limit value over which the fuel accumulation cannot withstand, an actual pressure on the fuel accumulating device may overshoot the target value during the increase of the fuel pressure in the fuel accumulating device. Hence, in the worst case, the fuel accumulating device will be broken since fuel pressure therein exceeds the limit value.
Further, if a target value of the fuel pressure in the fuel accumulating device is set near the lower limit value under which the engine cannot continue running, an actual pressure on the fuel accumulating device may undershoot the target value during the decrease of the fuel pressure in the fuel accumulating device. In the worst case, the diesel engine may be stopped. In the diesel engine, fuel is ignited when fuel and hot compressed air are mixed in the engine cylinder. However, in the state where the fuel pressure in the fuel accumulating device undershoots the target value while the pressure reducing valve is opened so as to discharge high pressure fuel from the fuel accumulating device, fuel is not sufficiently compressed for ignition.